News

New BJD test about to come into play

James Nason 29/01/2013

A new test soon to be introduced for Bovine Johne’s Disease carries hopes of speeding up diagnostic processes and reducing the time some properties will be required to spend under costly and disruptive quarantine restrictions.

However, the much-anticipated adoption of the ‘faecal polymerase chain reaction’ (PCR) test comes with a caution that it is far from a silver bullet that will simplify and accelerate herd assessment processes in all circumstances.

The benefits of the PCR test according to Biosecurity Queensland will include greater accuracy than the existing herd screening test – the ‘ELISA’ blood test – and much faster results than the faecal culture test.

The PCR can provide similar accuracy to the faecal culture test, but can return a result within two to three weeks, compared to the 12 weeks it takes to produce a faecal culture test result.

However, while it will reduce overall quarantime time for some trace-forward herds, this will apply in circumstances where only one herd survey test is required.  Many herds will still require two clean tests three to six months apart (see the seven testing scenarios outlined below).

While a positive PCR result will be interpreted as an indication an animal has BJD, a negative PCR result alone cannot guarantee a herd is ‘clean’, because, like the faecal culture test, it cannot pick up BJD in an animal that is infected but not shedding bacteria.

However, as the below scenarios provided by Biosecurity Queensland explain, negative results to paired PCR tests or combinations of PCR and histology tests will provide confidence that animals are not shedding BJD bacteria, and would allow movement restrictions on quarantined properties to be lifted.

The PCR test has recently been approved by the national Animal Health Committee (an arm of the Council of Australian Governments (COAG) system), and the Queensland Chief Veterinary Officer has also now approved its use in Queensland.

Biosecurity Queensland said the test is currently undergoing validation trials at its laboratory at Coopers Plains, Brisbane, in preparation for use in “the very near future”.
  
Until it is validated in Queensland, the department says faecal samples will be sent to the Elizabeth Macarthur Agricultural Institute (EMAI) in New South Wales for PCR testing.
  
According to advice provided to Beef Central by Queensland Chief Veterinary Officer Dr Rick Symons on Friday, PCR tests will augment the existing testing process in a number of ways, and has outlined seven testing scenarios to illustrate how results will be interpreted (outlined below).

The PCR test directly detects the genetic material of the causative BJD agent that may be present in the faeces. It takes much less time than the faecal culture test and is potentially more sensitive, according to Biosecurity Queensland.

The test can be performed within two to three weeks for a similar cost to faecal culture. 

“Depending on the circumstances, the reduced testing time to get a PCR result could result in a reduced quarantine period,” Dr Symons said.

“The faecal PCR will produce similar results to the faecal culture test, and may be more sensitive.”

In highly contaminated environments it will be possible to get a positive PCR result from ingested BJD bacteria, rather than bacteria shed from the animal being infected.

However, even in this case, it means that BJD is present on the property.

As is the case with faecal culture, a positive result in an individual animal using the PCR test will be deemed as an indication that the animal has BJD.

In most cases an animal returning a positive PCR test result would then be slaughtered and further samples collected, according to Biosecurity Queensland.

An infected animal early in the infection cycle will most likely test negative to either the faecal PCR or faecal culture test.

Dr Symons said a negative PCR result in itself “is not final”, as shedding can be intermittent and further sampling and laboratory testing may be required.

He said the test may also be used to determine the infection-status of a herd.

“BJD bacteria are not detectable by faecal PCR or culture from an individual animal that is not shedding,” Dr Symons said.

“Therefore, two faecal samples are taken three to six months apart to determine whether an animal is shedding.

“However if a herd is infected, it is likely that at least one of the infected animals is shedding at any one time and it is on this basis that some tests are applied on a herd basis rather than an individual animal basis.”

In addition to laboratory testing, there are a number of ways that properties that have received trace-forward animals may be assessed.

Each has advantages and disadvantages, so testing regimes are tailored according to individual circumstances.

Therefore, if a herd tested negative to the PCR test it would depend on the individual circumstances applying to the herd before it could be deemed to be free from BJD.

A summary of the main assessment options, provided by Biosecurity Queensland, follows:

1.      On-farm or abattoir slaughter of all trace-forward cattle, plus definitive testing by histology and tissue culture.
This is the most definitive method, but also means destroying the animals and in some cases animals tested by histology and tissue culture they may no longer be available. Culture takes up to three months to report results. If negative for all animals, the movement restrictions on the herd will be lifted.

2.      On-farm or abattoir slaughter of all trace-forward cattle, plus testing by histology and faecal PCR.
This is a new option recently approved. While it is possible for an animal to later return a positive culture result, if negative to these two tests, we can be confident that the animals were not shedding BJD bacteria, so the movement restrictions will be lifted.

3.      Paired faecal culture or faecal PCR test on all trace-forward animals, 3-6 months apart, with immediate removal (e.g. slaughter) of trace-forward animal(s) after second sampling.
This option applies where all trace-forward animals are still available. Testing is to demonstrate that the animal(s) are not shedding BJD bacteria and therefore have not presented a risk to the herd. The two tests are required as faecal shedding in infected animals can be intermittent.

It does not mean that they are not infected, but if these tests are negative the movement restrictions will still be lifted. This option allows greater scope for postponing removal for breeding or other purposes. The culture test takes some time, but if replaced by the PCR test the results should be available within 2-3 weeks.

4.      Paired faecal PCR test on all trace-forward animals, 3-6 months apart, plus isolation of these animals from the main herd.
Animals can be retained for breeding purposes and the movement restrictions on the rest of the herd can be lifted, provided the suspect animals remain isolated – that is separated from susceptible animals and subjected to movement restrictions.

Periodic monitoring of the suspect animals is required until they are eventually removed (e.g. through turn off to slaughter).

5.      Herd (or exposed group) sample test, designed to identify whether infection has spread into the herd.
This single test is used if some or all of the trace-forward animals are no longer present and can be done any time after two years after the date of last dangerous contact (this time delay is to allow recent infection to become detectable – the longer the period the better).

Further monitor tests may be advisable. Either faecal culture or faecal PCR can be used. Only adults (greater than two years old) are sampled and the sample size will depend on the size of the herd.  If all animals are negative, the movement restrictions will be lifted.

6.      Identification and preferential culling of at risk animals.
If there are a relatively small number of at risk or exposed animals on a property, culling of these may resolve the property status without the need for any testing.

7.      Combination of strategies.
Combined strategies may be used to cater for individual circumstances. For example, an extensive property where a significant number of the suspect bulls cannot be mustered but may still be alive on the property. Sample herd testing may be required over a two year period, as well as testing of any bulls that can be located, before movement restrictions can be lifted. Further monitor testing may also be required.

 

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